應用於PCB之微型化超高頻RFID標籤天線

Abstract

當RFID標籤嵌入在一個印刷電路板(PCB)時，因受限於現有電路之佈局，將大幅減少RFID標籤可使用的面積，但又必須保持一公尺左右的讀取距離，以結合實際生產線的使用情境。本論文提出運用PCB之多層結構及結合LC諧振與能量耦合的兩種概念，可有效減少天線面積，並使用裂環型諧振(Split Ring Resonator)和平面螺旋諧振(Planar Spiral Resonator) 兩種LC諧振形式並比較其優劣。本論文兩種天線形式皆為雙層結構並製作在厚度0.6mm之FR-4，其上層皆為蜿蜒式迴圈，作用為共軛複數阻抗匹配以及將能量耦合到下層輻射板來達成RFID標籤天線縮小化之目的；而下層則為LC諧振，一種為裂環型諧振(SRR)，其原理是由SRR的環型和開口產生電感和電容，而開口的地方使用交叉式樹狀結構產生較大之電容，達到小電感大電容的方式共振，另一種則為平面螺旋諧振，其原理是由螺旋狀的結構產生電感以及螺旋狀的間隙產生電容達成LC諧振，再透過一長條輻射區域輻射，透過數值模擬軟體(HFSS)模擬其天線得到的結果在頻段922~928MHz具有相當良好的共軛複數阻抗匹配，實作天線經實測結果顯示，平面螺旋諧振型式天線之面積僅需50mm2，在如此微小的RFID標籤天線又擁有1公尺的讀取距離，故此標籤天線非常具有實際應用價值。本天線的設計細節及實驗結果在論文中有詳細討論。

RFID tags embedded in a printed circuit board (PCB) have to feed themself into a limited space due to the preexisting onboard circuit elements, and for practical applications these tags have to support at least one meter reading distance. This paper employs multi-layer structure and proposes the concepts of the combination of LC resonance and energy coupling. This can effectively reduce the antenna area. In addition, this work also makes a comparison between the two kinds of LC resonators, the split ring resonator and the planar spiral resonant. The antennas presented in this paper are both implemented by the two-layer structure with the FR-4 substrate of the thickness 0.6 mm. The winding loop, on the top layer, is used for the complex-conjugate match and coupling the energy to the bottom layer to achieve RFID tag antenna size reduction purposes. While in the bottom layer the capacitance of LC resonator realized by the SRR is enhanced by adding the inter-digital capacitor tree structure at the gap of split ring. On the other hand, the planar spiral-type LC resonance in the bottom layer is present by the spiral inductance and the capacitance between the lines. The antenna radiates from the long strip which is connected directly to the spiral resonator. The proposed antenna is designed for RFID application, 922MHz~928MHz, and it occupies the footprint of 50 mm2 and has perfect complex-conjugate match. Moreover, the reading distance can still support one meter long. Therefore, this RFID tag proposed here is suitable for practical applications.